The difference between an experienced scientist and a novice in the art is not necessarily that the student makes mistakes where the experienced does not. Rather, the experienced anticipates those mistakes and plans for them; s/he knows when they will occur and how to design an experiment such that the mistake does not matter.
So, to illustrate, I always run asymmetric protein and nucleic acid gels; if I drop them I can tell which way around they are supposed to be. Or, to take a completely random example, when running western blots of multiple samples from two experimental cell types I will make one of the gels subtly different, just in case - oh I don't know - maybe I absent-mindedly ignore cough the labels cough on the soaking trays cough or something like that, then I will, tomorrow, still be able to interpret the experiment.
